A current sensing technique is known in which phase currents are detected by a single shunt resistance inserted in a direct current part of an inverter circuit when U-phase, U-phase and W-phase currents are detected for the purpose of motor control. A three-phase PWM signal pattern needs to be generated so that two or more phase currents can be detected within one period of a pulse width modulation (PWM) carrier (a carrier wave), in order that all the three phase currents may be detected by the use of the aforementioned system. A first conventional technique reference proposes that PWM signal pulses of the respective phases be shifted for the purpose of reliable execution of current detection.
However, when the pulses are simply shifted, a motor current presents a stepwise variation in synchronization with transition from a pattern, resulting in a problem of increasing a level of noise produced during drive of an electric motor. In view of this problem, a second conventional technique reference discloses a technique for setting arrangement of PWM signals so that the pattern does not change, thereby improving a current detection rate and suppressing increases in current ripple and accompanying noise.
In the above-mentioned second reference, however, for the purpose of suppressing noise increase, an improvement in the current detection rate is partially limited in a region where a modulation factor is higher. A comparison will now be made among current detection rates obtained by respective PWM signal generation methods. A minimum pulse width τ [s] required for detection of DC current depends upon dead time, delay time of a current detection circuit and the like. Furthermore, a minimum duty Dmin [%] required for current detection is obtained from the minimum pulse width τ and a PWM period T [s] by equation (1):Dmin=2τ/T×100  (1)
When the modulation factor is defined as a ratio of an inverter line voltage amplitude to a DC power supply voltage and τ=10 [μs] and PWM period is set to 100 [μs], FIG. 15 shows current detection rates according to modulation factors of systems. The current detection rate is calculated as a rate of a section in which two or more phase currents are detectable in one period by electrical angle.
Although a general triangle-wave comparison modulation (♦) has a large number of sections where the modulation rate is low so that current cannot be detected, the above-described first reference (▪) improves the phenomenon. On the other hand, in the reference (▴) rendering the fixed pulse placement of PWM signals variable or more particularly, in the above-described second reference, when a pulse placed in the center of carrier period is set to a phase indicative of maximum duty in three phases, the current detection rate is improved in the region where the modulation rate is high. However, this increases torque ripple, resulting in an increase in the drive noise of the motor.